Biodiesel is an alternative renewable fuel made from vegetable oils, fats, greases or other sources of triglycerides. It is a nontoxic and biodegradable substitute and supplement for petroleum diesel.
Biodiesel fuels typically comprise lower alkyl fatty acid esters, prepared for example by transesterifying triglycerides with lower alcohols, e.g. methanol or ethanol. A typical biodiesel fuel is the fatty acid methyl ester of rapeseed oil or of soy oil.
Biodiesel fuel and its preparation is taught for example in U.S. Pat. Nos. 5,578,090, 5,713,965, 5,891,203, 6,015,440, 6,174,501 and 6,398,707, the contents of which are hereby incorporated by reference.
One of the major problems associated with the use of biodiesel is its poor cold flow properties resulting from crystallization of saturated fatty compounds in cold conditions. A 20° C. reduction in cold filter plugging point is necessary for some biodiesel fuels to find utility in colder climates such as those of the United States and Europe in winter.
It is well known to add pour point depressants or cold flow additives to conventional petroleum-based fuel oil in order to improve it cold flow properties.
Long chain poly alkyl(meth)acrylates are a class of pour point depressant additives for petroleum-based fuel. These compounds are described, for example in U.S. Pat. Nos. 2,091,627, 2,100,993, 2,114,233 and 4,867,894.
Attempts have been made to apply the same long chain poly alkyl(meth)acrylates to improve the cold flow properties of biodiesel fuels.
For example, U.S. Pat. Nos. 6,203,585 and 6,391,996, herein incorporated entirely by reference, disclose a biodiesel fuel composition having a depressed pour point comprising a copolymer additive formed from long chain alkyl(meth)acrylate monomers.
Many different well-established methods are available for polymerizing these long chain poly alkyl(meth)acrylates. Most methods have the disadvantage that uncontrollable recombination reactions of initiator radicals may occur immediately after their formation with the effect that variable ratios between initiator radicals and stable free radicals are produced. Consequently, in some cases there is an inefficient control of the polymerization process.
Group Transfer Polymerization (GTP) is a well-established method for producing A-B block copolymers of defined structure from methacrylate monomers. Despite its wide applicability and usefulness the GTP method still has several drawbacks. The polymerization initiators used in this method, such as the silyl ketene acetals disclosed in U.S. Pat. No. 4,656,226, e.g. 1-trimethylsilyloxy-1-isobutoxy-2-methylpropene, are highly reactive and difficult to prepare in a multi-step synthesis. This necessitates the use of carefully dried and purified reactants, which limits this method in industrial applications operating on a large scale.
U.S. Pat. Nos. 5,763,548 and 6,407,187 disclose a controlled or “living” polymerization process of ethylenically unsaturated polymers, such as styrene or (meth)acrylates, by employing the Atomic Transfer Radical Polymerization (ATRP) method. This method produces defined oligomeric homopolymers and copolymers, including block copolymers. Initiators are employed, which generate radical atoms, such as .Cl, in the presence of a redox system of transition metals of different oxidation states, e.g. Cu(I) and Cu(II), providing “living” or controlled radical polymerization. U.S. Pat. No. 6,391,996 uses just such a system for the production of poly alkyl(meth)acrylates for biodiesel applications.
A general drawback of this prior art method is seen in the fact that the polymer chains prepared by ATRP contain halogen as terminal fragment, which has been transferred from the polymerization initiator. The content of halogen is generally undesirable in polymers. Halogen, especially chlorine and bromine, is subject to the removal as hydrogen halide depending on temperature, especially above 150° C. The double bond thus formed is subject to a reaction with atmospheric oxygen, which decreases the antioxidative resistance of the polymer. Moreover, hydrogen halide liberated from the polymer reacts with other functional groups present in the polymer, such as ester groups present in acrylates. Depending on the type of the polymer, chlorine is also removed in the form of a radical, which might initiate undesirable chain reactions in the polymer structure. The removal of halogen from the polymer structure, especially from the terminal position of the polymer chain, and its replacement with suitable substituents in a subsequent process step is described in U.S. Pat. No. 6,433,100. Another drawback to ATRP is the removal of the copper catalyst from the final product. Copper is a pro-oxidant and should be avoided for applications involving fuels and lubricants because it can catalyze rapid oxidation. In applications such as biodiesel, the presence of copper is especially concerning because biodiesel is generally less stable towards oxidation than conventional petroleum diesel.
U.S. Pat. No. 4,581,429 discloses a free radical polymerization process by the controlled or “living” growth of polymer chains. A specific process embodiment is the use of initiators of the partial formula R′R″N—O—X. In the polymerization process the free radical species R′R″N—O. and .X are generated. .X is a free radical group, e.g. a tert-butyl or cyanoisopropyl radical, capable of polymerizing monomer units containing ethylene groups. The monomer units A are substituted by the initiator fragments R′R″N—O. and .X and polymerize to structures of the type: R′R″N—O-A-X (A: polymer block). Specific R′R″N—O—X initiators mentioned are derived from cyclic structures, such as 2,2,6,6-tetramethylpiperidine, or open chain molecules, such as di-tert-butylamine. Recently some alternative polymerization regulators have been published. WO 98/30601 discloses heterocyclic >N—O—R compounds suitable for controlled polymerization processes. WO 98/13392 discloses open chain alkoxyamines, which are derived from NO-gas or from nitroso compounds. The advantage of these prior art polymerization methods over the ATRP-method is seen in the fact that no subsequent replacement of terminal groups of the polymer chains is needed.
The present inventors have found that the preparation of polymers or copolymers of alkyl(meth)acrylates via nitroxyl mediated controlled free radical polymerization (CFRP) provides (co)polymers of well controlled molecular weight and polydispersity without the problematic terminal groups or copper contamination produced by ATRP methods. Furthermore, the CFRP polymeric (meth)acrylates are effective as cold flow improvers for biofuels.